The invention relates to an improved method for decreasing the fluid loss or seepage loss from a drilling fluid.
A drilling mud system is designed to be a circulating system, hopefully, with all the fluid retained in the system itself. However, various types of losses do occur during the drilling operation. These losses occur on the surface due to storage, evaporation, etc. Other losses occur on the surface as a result of a certain amount of the mud adhering to the drill solids that are finally disposed of from the system. However, the greatest loss of fluid generally occurs down hole during a drilling operation. These losses can take several different forms. For example, large, gross loss of whole fluid (solids and liquid) is commonly referred to as lost circulation. Lost circulation can be anywhere from 100% to only a partial loss of returns down to 20% or 30%. At the other extreme is a loss of only the liquid phase which is generally referred to as fluid loss (liquid phase only). This loss occurs as the result of the actual filtration of the solids by the formation due to a differential pressure from the fluid column to the formation. A generalized category that is referred to as seepage loss occurs when there is a combination of whole loss of fluid coupled with filtration losses. These losses are small but continuous.
Up until about 1973 seepage loss was not considered an important cost of running an oil mud. This was due to the fact that the cost of diesel at that time was in the $0.30/gal range, or approximately $12.60/bbl. No. 2 diesel fuel, which is the most commonly available oil used in oil base muds, represented a very insignificant cost of the drilling fluid. However, as the price of oil increased to above $1.00/gal, even small losses over a period of time became important. Small losses of oil base mud drilling fluid over a period of usage as a result of seepage loss/fluid loss in oils muds can be a significant cost in running the already expensive oil mud systems.
With the advent of "relaxed fluid-loss" or low-colloid oil muds, even higher losses can and do occur. A recognized offset to this cost, however, is the potential reduction of time and oil muds used due to the increase in penetration rate.
Density is one of the most important mud property affecting penetration rate. For any given formation pressure, the higher the density, the greater will be the differential pressure and consequently, the greater the static chip hold down, and likelihood of bottom-hole and bit balling. Decreasing mud density decreases dynamic chip hold down, permitting faster rpm, and by decreasing pressure losses in the drill pipe, increases hydraulic horsepower available at the bit. Thus the lowest possible mud density should always be carried. Wherever possible, an underbalanced mud column should be used. In normally pressured formations, differential pressures should be kept no higher than necessary to establish a filter cake on unconsolidated sands.
Viscosity is another mud property that materially influences penetration rate. Low viscosity promotes fast rates mainly because of good scavenging of cuttings from the under bit. The relevant viscosity is the effective viscosity at the shear rate prevailing under the bit, not the plastic or funnel viscosity. Low viscosities are particularly important at high rotational speeds because of lower dynamic chip hold down. When the bit tooth strikes, the fractures are at first exceedingly small and the viscosity of the filtrate is probably the relevant factor, but as the chips move out, the viscosity of the mud becomes relevant.
Ideally, in a low-colloid oil mud it would be desirable to have maximum filtration at the face of the bit but zero fluid and seepage losses in the annulus side of the hole. It is usually necessary to compromise in utilizing a low-colloid oil mud by allowing the fluid loss to increase in the oil mud system to a desired level and then holding it at that level in order to achieve a maximum fluid loss at the bit face and yet minimize the seepage loss or fluid in the side of the hole.
During the drilling of thick sections of highly permeable formations, seepage losses and fluid can be excessive when low-colloid oil mud systems are used.
Suggestions have been made for controlling excessive losses in such systems. These include:
1. by-passing all solids removal equipment on a temporary basis and allowing drill solids to be retained until sufficient bridging particles accumulate and form a seal; PA1 2. adding bridging solids, such as fine nut hulls, mica, unmodified lignite, modified asphalts, etc.; and PA1 3. addition of a low concentration of filtration control additive for excessive filtration.
Typically, products such as fine rice and fine nut hulls have been used in oil mud systems, more from the fact that they do not add to the mud problems than from actually providing much assistance in loss control.
Historically, lost circulation, seepage loss, and fluid loss problems have been more difficult to control in oil mud systems than in water base muds because most common additives tended to drastically alter the properties of the oil muds. Cellulose base materials such as wood fibers, ground paper, cotton seed hulls, bagasse, etc., tend to preferentially water wet and can result in breaking the emulsion and "flipping" an expensive system. When this occurs, fluid loss becomes extremely high, weighting material water wets and settles out, water shows up in the filtrate, and other adverse effects occur. The system then has to be reconditioned. This usually results in the system being displaced, hauled into a plant and reconditioned at an extremely high cost. Ground plastics have been suggested and in some cases used, but again, not without problems. Some materials contain plasticizers and other additives that tend to also act as emulsion breakers at low concentrations.
In other cases products such as cellophane, although plastic in nature, have a peculiar wetability and can result in adsorption of water from the emulsion creating water wetting problems.
Many of the modified asphalts are effective as both seepage loss additives (when sized) or as fluid loss additives due to their colloidal nature. These products generally tend to increase the viscosity of the oil phase and add to the colloidal concentration of particles, resulting in decreased penetration rates. Moreover, relatively high concentrations of these products are needed in order to be effective fluid loss or seepage loss additives.
A secondary effect of many of the conventional loss control additives is their ability to absorb the wetting agents from the oil mud and thus deplete the system of the excess required to maintain a stabilized system.
Sometimes the effect Of addition of loss control additives to the low-colloid oil systems are not evident immediately. Both time and temperature play a significant role in effect of such additives. Thus, the addition of loss control additives to these oil mud systems should be selected with care.
Another common problem with many conventional loss control additives, particularly in the seepage loss range, is particle size. Typically, a treatment to the system is made. The material is circulated down hole one time, comes back to the surface and is taken out by even coarse shaker screens, unless the solids-control system is bypassed, and this causes other problems. Not only does additional material have to be added for loss control, but additional emulsifiers and wetting agents have to be added as a result of their being removed by adsorption/absorption on the loss control additives that are being discarded. Thus, an inexpensive loss control additive can become very costly in a low-colloid oil mud if it is not designed specifically for such a system and if it is not applied correctly.
In order to combat or prevent lost circulation, it has been common in the past to add any number of materials to the drilling fluid which act to reduce or prevent flow of the drilling fluid outwardly in a porous stratum thereby arresting a lost circulation condition. These materials are commonly referred to as lost circulation materials. Such prior known lost circulation materials include fibrous, flake, and granular materials. Representative of the organic natural products or modifications thereof that have been disclosed for use as lost circulation materials include nut and seed shells or hulls (pecan, almond, walnut, peach, brazil, coconut, peanut, sunflower, flax, cocoa bean, cottonseed, rice, linseed); crude pectate pulp; feathers; citrus pulp; beet pulp; peat moss fibers; jute; flax; mohair; lechuguilla fibers; cotton; cotton linters; wool; paper; wet-strength paper; sugar cane; bagasse; bamboo; corn stalks; sawdust; straw; wood fiber; cedar fiber; bark chips; cork; popped popcorn; dehydrated vegetable matter (suitably dehydrated carbohydrates such as citrus pulp, oatmeal, tapioca, rice grains, potatoes, carrots, beets, and various grain sorghums); the ground woody ring portion of corn cobs; whole ground corn cobs; hydrophobic, organophilic, water-wettable fibrous materials such as treated cotton, dried bagasse, and dried peat fibers; and specific mixtures of these materials. Many assorted inorganic materials have also been used as lost circulation additives.
Seepage losses can occur to any type of loss zone and in any type of formation when the particles in the mud are not fine enough to complete the seal. It has been established that the maximum allowable drilling fluid loss is on the order of 1 bbl/hr [0.16 m.sub.3 /h], as measured in the mud pit at the surface. Remedial measures should be taken when the mud loss exceeds 1 bbl/hr [0.16 m3/h].
Many of these problems associated with oil-based muds are also present in the more frequently used water-based muds. Further, in the rotary drilling of wells with aqueous base drilling fluids, various problems associated with the lubricating characteristics of the drilling fluid may occur, such as slow drilling rate, excessive drill pipe torque and drag, differential sticking, etc. See for example U.S. Pat. No. 4,356,096, incorporated herein by reference. This patent discloses that liquid lubricating additives can be sorbed onto certain hydrophobic, organophilic, water wettable absorbents to filter out or deposit in the wall cake on the sides of the borehole. This results in an increased concentration of the liquid lubricant in the wall cake where it is needed. The liquid lubricant can be Dremixed with the sorbent before adding the resulting solid lubricating additive to the drilling fluid.
Materials generically referred to as gelling materials ("gelling agent"), thinners and fluid loss control agents are also typically added to aqueous based drilling fluid formulations. Of these additional materials, each can be added to the formulation in a concentration as rheologically and functionally required by drilling conditions. Typical of gelling agents used in aqueous based drilling fluids are high molecular weight polymers such as PHPA, biopolymers, bentonite and salt gel. Examples of biopolymers are guar gum, starch and the like.
Similarly, it has been found beneficial to add lignosulfonate as thinners for aqueous based drilling fluids. Typically lignosulfonates, modified lignosulfonates, polyphosphates and tannins are added. In other embodiments low molecular weight polyacxrylates can also be added as thinners. Thinners are added to a drilling fluid to reduce flow resistance and gel development. Thinners accomplish this by reducing the mud viscosity, sometimes referred to as thinning the mud or reducing the gel strength. Other functions performed by thinners include to reduce filtration and cake thickness, to counteract the effects on salts, to minimize the effects of water on the formations drilling, to emulsify oil in water, and to stabilize mud properties at elevated temperatures.
It is object of the present invention to provide an additive which is very effective as a seepage loss control agent in both water base and oil base well working fluids.
It is another object of this invention to provide well working compositions having a low seepage loss.
Still another object of this invention is to provide a method of decreasing the seepage loss from water base and oil base well working fluids.
A further object is to provide a seepage loss reducing additive that also reduces the mud viscosity.
These and other objects of the invention will appear to one skilled in the art as the description thereof proceeds.